1. Field of the Invention
The present invention relates to a three-dimensional photonic crystal, more particularly, though not exclusively, the present invention relates to photonic crystals having designed, defects and/or refractive index periodicity.
2. Description of the Related Art
Yablonovitch proposed the concept that the transmission/reflection characteristics of an electromagnetic wave can be controlled using a structure smaller than the wavelength of the electromagnetic wave (Physical Review Letters, Vol. 58, pp. 2059, 1987). According to this document, a periodic structure smaller than the wavelength can control the transmission/reflection characteristics of the electromagnetic wave. Thus, the transmission/reflection characteristics of light can be controlled when the wavelength of the electromagnetic wave is close to the periodicity of the structure. A photonic crystal can be such a structure.
It has been suggested that a reflecting mirror having a reflectance of 100% (lossless) in a certain wavelength region can be manufactured. This concept that facilitates a reflectance of near 100% in certain wavelength regions, results in a frequency range with a reduced transmissive wavelength power, which is referred to as a photonic band gap, as compared with the energy gap in a conventional semiconductor. Furthermore, a three-dimensional fine periodic structure can provide the photonic band gap for incident light from any direction. This is hereinafter referred to as a complete photonic band gap. The complete photonic band gap can have various applications (e.g., reduced spontaneous emission in a light-emitting device).
For example, a point defect or a linear defect in the three-dimensional photonic crystal can provide a resonator or a waveguide in accordance with a desired wavelength of the photonic band gap. A point defect resonator utilizing the photonic band gap can trap light in a very small region and control the emission pattern of light, where the frequency of the emitted light can lie in the bang gap region. This can achieve an increased performance light-emitting device that efficiently emits light at a desired wavelength. When a point defect resonator is made of a luminescent material, the luminescent material can be excited by any excitation method to generate laser oscillation (U.S. Pat. No. 6,392,787).
A structure that can achieve the complete photonic band gap in a wider wavelength region can facilitate extending the operating wavelength region of such a functional device. Some structures having a photonic band gap have been proposed (U.S. Pat. No. 6,392,787, U.S. Pat. No. 6,134,043, Applied Physics Letters, Vol. 84, No. 3, pp. 362, 2004). FIGS. 33A to 33F illustrate three-dimensional periodic structures that allege to achieve the complete photonic band gap. They are a diamond structure, a woodpile structure, a helical structure, a three-dimensional periodic structure, an inverse structure of the three-dimensional periodic structure, and a diamond woodpile structure.
The photonic band gap in the three-dimensional photonic crystals described above can be controlled by changing the lattice period of the three-dimensional photonic crystals. For example, a larger lattice period shifts the wavelength band of the photonic band gap toward a longer wavelength, and a smaller lattice period shifts the wavelength band of the photonic band gap toward a shorter wavelength.
Noda et al. (Nature, Vol. 407, p. 608, 2000) alleges the control of an operating wavelength by lattice period modulation in an optical multiplexing/demultiplexing circuit (add-drop optical circuit) using a two-dimensional photonic crystal. The optical multiplexing/demultiplexing circuit is an optical input/output circuit that has a (add) function of adding a new wavelength to a medium through which a plurality of wavelengths propagate and a (drop) function of extracting only a certain wavelength from the medium. The photonic crystal is expected to reduce the size of this circuit. This literature alleges that almost the same drop efficiency could be obtained in a plurality of wavelengths by modulating the lattice period to tune the operating wavelength of a waveguide and a resonator to a desired wavelength. The structure in which two-dimensional photonic crystals of different lattice periods are arranged is called an in-plane heterostructure. This example demonstrates that the control of the wavelength band of the photonic band gap can be used for creating an optical nanodevice, which can use a photonic crystal.
However, such a structure having a modulated lattice period cannot be applied directly to the three-dimensional photonic crystal. While it is possible to control the photonic band gap by modulating a lattice period even in the three-dimensional photonic crystal, an incommensurate structure may occur at an interface where the lattice period varies, as shown in FIGS. 34A and 34B. In particularly, it is difficult to produce the three-dimensional structure, because inconsistent lattice periods occur in the x-axis, the y-axis, and the z-axis direction. For example, in a layer-by-layer structure in which layers are stacked one after another (e.g., the woodpile structure), the lattice period varies in the lamination direction. Thus, it is difficult to utilize the conventional repetition of structure patterning by electron beam lithography and the lamination, a conventional wafer-fusion technique, or a conventional nanoimprint process without any modification.
Accordingly, it can be difficult to have a conventional three-dimensional photonic crystal operating at a plurality of design wavelengths in which the photonic band gap can be adjusted to a desired wavelength band without changing the lattice period.